The summer of superheroes continues with Captain America, the latest Marvel Comics character to go big-screen.

We meet the unlikely kid who’ll become the cap’n, Steve Rogers, back in the early days of World War II. Steve’s a reed-thin, 98-pound weakling with a strong red-white-and-blue heart--but a list of health problems that takes up a full page on his enlistment form. Not surprisingly, Uncle Sam turns him down...again and again.

How scrawny Steve eventually transforms into secret-weapon studcake is part of the fun of “Captain America,” which embeds a rollicking superhero backstory into a gung-ho, retro-glow WWII yarn about a mad Nazi scientist (Hugo Weaving), his storm-trooper minions and a diabolical plan to out-Hitler even Der Führer himself.

Stanly Tucci plays the defected German bioengineering genius whose research gives little Stevie the boost he needs to become a U.S. Army superhero. Tommy Lee Jones is the crusty colonel who learns how to handle his unique new recruit. British actress Hayley Atwell brings brains as well as beauty to the role of special military agent Peggy Carter, for whom defeating the Nazi menace will mean she can finally let down her hair and go dancing with America’s splashiest battlefield star.

But the real star of the show is Chris Evans, the relative newcomer who plays Rogers. His Captain America is a hero with heart and a sense of humanity that rises above the slam-bang special effects and pulpy myth ‘n’ science subplot. As Rogers is told on the eve of his transformation, he needs to be not only a great soldier, but also “a good man.”

Director Joe Johnston, who previously steered “Jumanji,” the 2010 remake of “The Wolfman,” and “Jurassic Park III,” confidently walks the line between faithful comic-book iconography and contemporary action-movie splash, ultimately setting up Marvel’s next movie, “The Avengers,” which will bring together a federation of superheroes---including Captain America, Thor and Iron Man---next summer.

And Johnston uses his special effects with admirable restraint, a refreshing change from most superhero-movie blowouts of computer-generated bombast. And less is more: The explosions, gunfire and chase scenes are all the more exciting because they’re not coming at you every five minutes.

The movie pulls off an interesting twist in its closing act, but its neatest trick is practically invisible. An impressive bit of Hollywood digital “erasure” trickery lets Evans play Rogers as both the pre-Captain string bean as well as the muscle-bound, shield-wielding crusader he becomes.

In a summer of oppressive temperatures, political mire and numbing economic ache, it feels good to see someone swathed in the cool colors of the good ol’ U.S. of A. bravely buckling down to make the world a better place, one bully at a time. We could all use a rah-rah dose of the get-’er-done movie mojo Captain America is peddling.